A current mirror is typically assembled from two transistors such as bipolar transistors or MOSFET transistors. For example, a current mirror can be constructed from two PNP transistors Q1 and Q2, where the collectors of the transistors Q1 and Q2 are commonly connected to a voltage potential such as ground, and the emitters of the transistor Q1 and Q2 are tied to a voltage potential V.sub.CC. The bases of the transistors Q1 and Q2 are connected in common with the collector of the transistor Q1 generally acting as an input current path while the collector of the transistor Q2 effectively acts as an output current path. Connecting the base and collector of the transistor Q1 results in the transistor Q1 operating in its active region and accepting any desired level of current.
Sinking a current from the collector of the transistor Q1 "programs" the current mirror and causes a voltage difference V.sub.BE between the base and emitter of the transistor Q1. Since the transistors Q1 and Q2 have the same base bias potential and relative base to emitter potential difference, the transistor Q2 is thereby programmed to source or "mirror" the same current through its collector. The current sourced by the transistor Q2 depends on the relative transconductance of the transistors Q1 and Q2. If the transistors Q1 and Q2 are well matched, the output current path of the current mirror will draw a current level the same as that of the input current path. In that case, the ratio of input to output current is equal to one. The ratio of the input to output current can be selected based on the ratio of the transconductances of the transistor Q1 and Q2.
In conventional mirrors however, the currents in the input and output current paths of the mirrors do not mirror correctly because the voltage at one current path can be different from the voltage at the other current path. For example, when the voltage at the output current path varies, the voltage at the input current path remains fixed, causing an imbalance in the relative voltage level between the two current paths. The change in the output voltage causes the output current to vary due to the finite output impedance of the current mirror. In the above example, a change in the collector voltage of the transistor Q2 in the output current path at a given current level causes a variation in V.sub.BE for the transistor Q2 due to the "Early" effect. This is because the curve of collector current versus collector-emitter voltage V.sub.CE at a fixed base-emitter voltage V.sub.BE is not flat. As such, the output current might vary substantially over the range of the output voltage of the mirror at the collector of the transistor Q2.
To alleviate this problem, some current mirrors include fixed biasing means for keeping the output voltage at a fixed level. For example, such a current mirror can be constructed from three PNP transistors Q1, Q2 and Q3, where the bases of the transistors Q1 and Q2 are connected in common with the collector of the transistor Q2, and the base of the transistor Q3 is connected to the collector of the transistor Q1. The collector of the transistor Q2 is connected to the emitter of the transistor Q3, and the emitters of the transistors Q1 and Q2 are connected to a voltage potential such a V.sub.CC. As such, the transistor Q3 keeps the collector of the transistor Q1 fixed at two diode drops below V.sub.CC, circumventing the Early effect in the transistor Q1. Sinking a desired input current through the collector of the transistor Q1 causes the transistor Q2 to mirror an output current through its collector. Both the current-determining transistors Q1 and Q2 have fixed collector-emitter drops. However, a major disadvantage of such current mirrors is that although the currents in the input and output paths remain balanced, the voltage at the input and output paths cannot vary, and only coupling of currents is permitted.
There is, therefore, a need for a balancing controller for balancing mirror currents in a current mirror while allowing the relative voltage on the current paths of the mirror to vary.